In a packet-switched network, the data to be transmitted take the form of packets processed by items of network equipment until they reach their destination. All of the packets to be transmitted constitute a datastream.
An example of technology used in packet-switched networks to convey data packets is the MPLS (MultiProtocol Label Switching) technology. The MPLS technology proposes adding, in the header of the data packets, one or more labels containing information allowing an item of network equipment to determine the next item of equipment through which a packet is to travel in order to reach its destination. The MPLS technology is described in greater detail in a document of the IETF (an Internet standardization group called the Internet Engineering Task Force), referenced RFC 3031 (RFC meaning “Request For Comments”).
However, the MPLS protocol makes it possible to process only packets that comply with the IP (Internet Protocol) protocol.
In order to alleviate this drawback, the PWE3 (Pseudo Wire Emulation Edge to Edge) standardization group of the IETF defines a pseudo-wire concept making it possible to emulate a point-to-point link between two items of equipment of a packet-switched network based on the IP/MPLS technology. Such pseudo-wires, defined in document RFC 3985 under the term “pseudo-wire”, allow the transmission of data packets that do not comply with the IP protocol, such as for example data packets complying with the ATM protocol.
With reference to FIG. 1, a pseudo-wire pw1 is established between a first item of terminal equipment T-PE1 placed on the edge of a packet-switched network PSN and a second item of terminal equipment T-PE2 also placed on the edge of the PSN network. A first link L1 constituting the pseudo-wire pw1 is established between the first item of terminal equipment and an item of switching equipment S-PE of the PSN network. A second link L2, also constituting the pseudo-wire pw1, is, for its part, established between the item of switching equipment S-PE and the second item of terminal equipment T-PE2. The first item of terminal equipment T-PE1 is called an item of input terminal equipment and constitutes a first end of the pseudo-wire pw1 and the second item of terminal equipment T-PE2 is called an item of output terminal equipment and constitutes a second end of the pseudo-wire pw1. Once the pseudo-wire pw1 is established, the item of input terminal equipment T-PE1 transmits a datastream routed via the pseudo-wire pw1 to the item of output terminal equipment T-PE2. Such a pseudo-wire pw1 is defined in greater detail in document “An Architecture for Multi-Segment Pseudo-Wire Emulation Edge-to-Edge. draft-ietf-pwe3-ms-pw-arch-03.txt”. Such a pseudo-wire allows a two-way transmission of the datastream. Therefore, in a first transmission direction, the first item of terminal equipment is an item of input terminal equipment, and in a second transmission direction, the first item of terminal equipment is seen as an item of output terminal equipment.
In order to ensure the continuity of service in the event of failure of the item of input terminal equipment, the PWE3 task force proposes a solution consisting in duplicating the first pseudo-wire, established between the item of input terminal equipment and the item of output terminal equipment, with a second pseudo-wire serving as a backup pseudo-wire so that, when the item of input terminal equipment forming one end of the first pseudo-wire fails, the data packets are routed by means of the backup pseudo-wire of which one end consists of a different item of input terminal equipment.
Thus, with reference to FIG. 2, a first pseudo-wire pw1 is established between a first item of input terminal equipment T-PE1 placed on the edge of a packet-switched network PSN and an item of output terminal equipment T-PE2 also placed on the edge of the network PSN. A first link L1 forming the pseudo-wire pw1 is established between the item of input terminal equipment T-PE1 and an item of switching equipment S-PE of the network PSN. A second link L2, also forming the pseudo-wire pw1, is established between the item of switching equipment S-PE and the item of output terminal equipment T-PE2. A second pseudo-wire pw2 is then established between a second item of input terminal equipment T-PE3, constituting a first end of the pseudo-wire, and the item of output terminal equipment T-PE2 constituting a second end of the pseudo-wire. A first link L3 forming the second pseudo-wire is established between the second item of input terminal equipment T-PE3 and the item of switching equipment S-PE and a second link L4, also forming the second pseudo-wire, is established between the item of switching equipment S-PE and the item of output terminal equipment T-PE2.
In order to ensure continuity of service, the item of output terminal equipment T-PE2 comprises means for detecting a failure of the links L1 or L2 or of a failure of the first item of input terminal equipment T-PE1 and means for switching the datastream from the first pseudo-wire pw1 to the second pseudo-wire pw2.
When a failure occurs in the first item of input terminal equipment T-PE1, said failure is detected in the item of output terminal equipment T-PE2, for example based on a message indicating the appearance of a failure transmitted over the network PSN to the item of output terminal equipment T-PE2. Once informed of the failure, the item of output terminal equipment T-PE2 requests the switchover of the datastream from the first pseudo-wire pw1 to the second pseudo-wire pw2. The data are then routed from the second item of input terminal equipment T-PE3 to the item of output terminal equipment T-PE2. The result of this is a lengthening of the restoration time in the event of failure of an item of input equipment having a negative impact on the quality of service. Moreover, such a solution has the drawback of consuming network resources such as, for example, processing resources in the items of equipment (storage memory capacity, computing capacity, etc.), the signaling streams for the purpose of establishing the two pseudo-wires, the bandwidth, notably between the item of switching equipment and the item of output terminal equipment.